Chemical treatment of water to obviate the dangers of microbiological contamination often requires the addition of oxidizing agents, for example, alkaline oxidizers such as calcium hypochlorite, liquid bleach (sodium hypochlorite) lithium hypochlorite or the like. While these compounds are highly effective water sanitizers, a problem concomitant with their use is the precipitation of carbonate salts, most notably calcium carbonate, on or in the oxidizer feed or injection equipment. Furthermore, chemicals which are commonly used to control the fouling effects of calcium carbonate scale are susceptible to oxidation which contributes additional undesirable byproducts to the water, e.g. phosphates.
Carbon dioxide is generally present in varying amounts in most natural waters, and is found to exist as several distinct species. As illustrated in the following graph, the percentages at which differing species exist varies as a function of solution pH. Water having a pH of less than 7.5 avoids carbonate alkalinity. At a pH of less than about 8.3, the carbon dioxide based alkalinity is in the form of bicarbonate (HCO.sub.3.sup.--) ions. Many common oxidizers raise the pH of water being treated. For example, in the case of calcium hypochlorite, it is common for water treated with this oxidizer to achieve a pH of greater than 10. As the pH rises above about 8.3, the alkalinity begins converting to the carbonate (CO.sub.3.sup..dbd.) form. Compounding the problem is the release of high concentrations of calcium which assure calcium carbonate scale formation. In the case of liquid bleach, a pH of greater than 12 occurs which induces calcium carbonate formation at injection points, thus reducing chemical delivery. ##STR1##
In the case of alkaline oxidizers the pH is very high, which is the result of a high presence of hydroxide based alkalinity (OH.sup.--) . Upon contacting water, for example in a water treatment process plant, the alkaline oxidizer causes the localized pH at the point of contact to rise. Bicarbonate alkalinity in the water then reacts with the hydroxide alkalinity, forming a carbonate ion and water in accordance with the following relationship: EQU HCO.sub.3.sup.-- +OH.sup.-- .fwdarw.CO.sub.3.sup..dbd. +H.sub.2 O
At this point the carbonate becomes available to combine with calcium forming a precipitate. The active formation of calcium carbonate in the water induces the formation of a crystal lattice structure at various points within the system, restricting waterflow and promoting or accelerating the failure of critical parts of the system.
Various classes of polymer compounds are known to have utility in impeding the progress of crystal lattice formation. Among these are polyepoxysuccinic acid (PESA) compounds and polymers of maleic and acrylic acids, the use of which is disclosed in the following prior art patents:
U.S. Pat. No. 4,654,159 teaches a method of forming a PESA compound which is useful as a scale control agent.
U.S. Pat. No. 5,062,962 describes a method of controlling scale formation by the addition of substoichiometric levels of PESA to circulating industrial cooling water systems where water from natural sources is employed as the cooling medium for heat exchangers.
U.S. Pat. No. 5,518,629 discloses a method of treating water to inhibit scale formation by utilizing a substoichiometric amount of a scale inhibitor in combination with a substituted alkylpolycarboxylate.
U.S. Pat. No. 4,087,360 teaches a method of scale inhibition utilizing a combination of a calcium hypochlorite and a proportion of a polyacrylic acid compound.
The problem with the prior art processes is that they have consistently predicated their choice of treatment ratios by utilizing the water's calcium level as a controlling parameter. By so doing, the prior art processes have been limited to the feeding of only substoichiometric levels of compounds, e.g. PESA or polymaleate compounds, in open recirculating systems; whereby scale inhibition is taught up to an LSI of only about 3.5.
If a process could be devised which optimized the active polymer utilization, then effective scale inhibition could be achieved at LSI values substantially in excess of those taught by the prior art.